Recently, engineering development using a terahertz wave is prosperous. In particular, substances can be analyzed using spectra in a frequency band of 30 GHz to 30 THz inclusive which are obtained by a spectrophotometrical method.
In such a technical situation, a technique of performing a DNA analysis using that a terahertz wave propagation characteristic of a transmission line changes by producing the terahertz wave transmission line on a substrate, and dropping and drying a DNA aqueous solution on the transmission line is disclosed in Phys. Med. Biol. 48, 3625 (2003) (hereinafter referred to as “non-patent document 1”). In non-patent document 1, it is used that values of a dielectric constant to a terahertz wave differ between a single strand DNA and a double strand DNA. Thereby, it can be inspected from difference between propagation characteristics of a terahertz wave whether a DNA is a single strand or a double strand.
As mentioned above, analysis, detection, identification, and the like of a substance can be performed using a terahertz wave by obtaining optical characteristics, such as an absorption coefficient, and a complex refractive index of the substance, from a change of a propagating state.
By the way, in the method of non-patent document 1, time delay is generated using a return optical system and a Littrow reflector. A time waveform of a propagating terahertz wave can be obtained by this time delay.
Generally, measurement of a terahertz wave is performed by acquiring a time waveform of the terahertz wave by terahertz time domain spectroscopy. Heretofore, a unit of changing an optical path length of probe light propagating is used. Thereby, timing of detecting a terahertz wave in the detecting unit (the detecting unit is irradiated with a pulse-like laser beam (probe light)) can be changed (time delayed). At this time, timing of arriving at the detecting unit (generated in the generating unit) is constant.